A technique that is disclosed in Patent Literature 1 [PTL1], cited below, is known as a technique that is related to an electrical power saving for a storage system for instance.
Patent Literature 1 discloses a storage system that is provided with a plurality of magnetic disk apparatuses. For the storage system, a magnetic disk in a magnetic disk apparatus to which an access is not executed for a certain period of time is made to be in a spin down state. By this configuration, an electrical power saving for a storage system can be carried out.    [PTL 1] Japanese Patent Application Laid-Open Publication No. 2000-293314.
A technique that is called Thin Provisioning (also referred to as Dynamic Provisioning) is known. In Thin Provisioning, one or a plurality of virtual logical volumes (virtual volumes) correspond to one or a plurality of substantive storage areas (hereafter referred to as pools) based on a physical storage device group, respectively. The virtual volume is configured by a plurality of virtual storage areas (virtual pages), and the pool is configured by a plurality of substantive storage areas (real pages). For instance, in the case in which a storage system receives a write request that specifies a virtual volume from a host, the storage system allocates a real page in a pool that corresponds to the virtual volume that is specified by the write request to a virtual page that is specified by the write request. The write target data that complies with the received write request is then written to the allocated real page.
An electrical power saving technique that is disclosed in PTL 1 is applied to a storage system to which Thin Provisioning is applied. In the case in which a physical storage device that is in an electrical power saving state is a physical storage device that is a basis of a pool (hereafter referred to as a pool storage device), it is thought that an effect of an electrical power saving is low. More specifically, one of the following (1) and (2) is considered: (1) a pool storage device is not likely to be accessed for a certain period of time; and (2) even in the case in which the pool storage device is made to be in an electrical power saving state since the pool storage device is not accessed for a certain period of time, the electrical power saving state is canceled immediately due to an access. This is because even in the case in which any one of virtual volumes that are associated with a pool is accessed, any one of pool storage devices that are a basis of the pool is possible to be accessed.
An object of the present invention is to improve an effect of an electrical power saving for a storage system to which Thin Provisioning is applied.